1. Field of the Invention
The present invention relates to improvements in a boiler of the type that consists of a furnace, a tunnel section at the upper portion of the furnace and a rear gas duct. The gas duct is divided into a plurality of passages. Reheaters and superheaters are provided separately in the respective passages, and flow rates of combustion gases flowing through the respective passages are made adjustable.
2. Description of the Prior Art
The type of boilers of the prior art mentioned above will be described with reference to FIGS. 5 and 6.
First, in a boiler illustrated in FIG. 5, a rear gas duct is provided with two partition walls 52 which intersect with side walls 51 of the rear gas duct perpendicular thereto. Thus, the rear gas duct is divided into three passages. A low-pressure reheater LP, a high-pressure reheater HP and a superheater SH are disposed in these respective passages in sequence starting from the passage nearest to a tunnel section 53. Dampers are disposed in the respective portions of the three divided passages proximte the lower ends of the partition walls 52.
Combustion gas coming from a furnace 54 passes through the tunnel section 53, and thereafter enters the rear gas duct. At this time, the combustion gas passes through only the passage in which the superheater SH is disposed. If the dampers are closed in the passages where the low-pressure reheater LP and the high-pressure reheater HP are disposed, respectively. Combustion gas passes through the passages where the low-pressure reheater LP and the superheater SH are disposed, respectively, if the dampers are closed in the passage in which the high-pressure reheater HP is disposed. In other words, if there is a passage in which the dampers are closed, the combustion gas does not flow through that passage but flows through the passages where the dampers are opened. Moreover, even if the dampers are opened, the flow rate of the combustion gas would vary depending upon a degree of opening of the dampers.
In a boiler shown in FIG. 6, a rear gas duct is provided with a partition wall 62 which intersects side walls 61 of the rear gas duct perpendicular thereto. Thus, the rear gas duct is divided into two passages. Furthermore, two partition walls 63 are provided parallel to the side walls 61 of the rear gas duct, to divide the passage that is nearer to the tunnel section 64 into three passages. In the passage positioned at the center of the passage rearer the tunnel section 64 a high-pressure reheater HP is disposed; low pressure reheaters LH are disposed in the passages positioned close to the side walls 61 of the rear gas duct, respectively. A superheater SH is disposed in the remaining passage. In the respective passages where the low-pressure reheater LP, the high-pressure reheater HP and the superheater SH are disposed, dampers are disposed proximate the lower ends of the partition walls 62 and 63.
Combustion gas coming from a furnace 65 passes through the tunnel section 64, and thereafter enters the rear gas duct. At this moment, the combustion gas passes through only the passage in which the superheater SH is disposed if the dampers in the passages where the low-pressure reheater LP and the high-pressure reheater HP are disposed are closed. The combustion gas passes through the passages where the low-pressure reheater LP and the superheater SH are disposed if the dampers in the passage in which the high-pressure reheater HP is disposed are closed. In other words, if there is a passage in which the dampers are closed, then the combustion gas does not flow through that passage but flows through the other passages where the dampers are opened. In addition, even if the dampers are opened, the flow rate of the combustion gas would vary depending upon a degree of opening of the dampers.
The boiler shown in FIG. 5 has the following shortcomings. The high-pressure reheater HP and the superheater SH are formed by extending heat transfer tubes in parallel to the side walls 51 of the rear gas duct and bending them downwards in the proximity of the walls so as to take a zig-zag path. The widths of the passages where the high-pressure reheater HP and the superheater SH are disposed are narrow. Hence, a proportion of a straight tube portion disposed in the passages as small, and the portion of the heat transfer tubes that is effectively available as a heat transfer surface is reduced. Upon fabricating the high-pressure reheater HP and the superheater SH the number of steps of bending tubes is increased, thus an amount of work is increased. Furthermore, when a gas duct evaporator is disposed on the downstream side of the high-pressure reheater HP, since inlet communication tubes of the gas duct evaporator are small in diameter and large in number, the arrangement thereof becomes complex. Or else, if the heat transfer tubes are extended in parallel to the widthwise direction of the furnace rather than in parallel to the side walls 51, then the lengths of the tubes would become long and the number of tubes would become small. Consequently, a pressure loss becomes excessively large.
On the other hand, the boiler shown in FIG. 6 has the following. Since the low-pressure reheater LP system is not disposed in a passage through which all of the combustion gas flows, the necessary heat transfer surface area is excessively increased. Moreover, since the low-pressure reheater is disposed in a passage divided along the widthwise direction of the furnace, a number of elements thereof becomes small and an increase in pressure loss occurs.